Photoelectric cell having a light sensitive electron emissive cathode



R. J. HERBOLD mum ITm

Oct. 20, 1953 i I I III/III Ir J Herbol zi Rober (Ittorneg Patented Oct.20, 1953 PHOT OELECTRI C CELL HAVING LIGHT SENSITIVE ELECTRON EMISSIVECATH- ODE Robert J. Herbold, Denver, Colo., asslgnor to Lafayette M.Hughes, Denver, 0010.

Application May 25, 1951, Serial N 0. 228,267 4 Claims. (Cl. 313-94)This invention relates to photoelectric cells having light sensitiveelectron emissive cathodes.

A photoelectric tube of the types shown in my prior Patents #2,489,220and #2,489,222 of November 22, 1949, comprises a suitable anodeassociated with a cathode having a surface that is sensitive to lightand capable of emitting electrons when light-activated. The emission ofelectrons from a saturated light sensitive cathode is proportional,other conditions being controlled, to the area of the sensitized surfacewhich is activated by the light rays. These constructions also comprisean opaque shield or one made of a suitable light filtering medium soarranged in front of and spaced from the cathode that as the angularityof an impinging light ray is varied, the activated area of the cathodeis varied, and thus the total electron emission is determined by thelight angularity.

This type of photo cell, which has various uses as well as those definedin said atents, may depend for its accuracy upon the degree ofuniformity of the light sensitive coating on the cathode. It, however,has been found that it is difficult to coat a cathode having anextensive fiat or cylindrical surface with sufficient uniformity to givethe desired accuracy, and standard procedures have resulted in someareas of the plate being less sensitive than others.

It is the primary object of this invention to provide a photo cell witha light sensitive electron emissive cathode which has a high degree ofuniformity of light sensitivity throughout its effective area and whichmay be readily and economically manufactured and will provide asatisfactory photo action. Other objects will be apparent in thefollowing disclosure.

In accordance with my invention, I make a cathode for a photo cell bycoating a wire with a suitable light sensitive electron emissivematerial and form the required cathode area of this wire. Since the wireis small in diameter as compared with its length and the total cathodearea, any variation in the uniformity of coating at a given point on thewire is negligible in its net result.

In the drawings illustrating one embodiment of this invention:

. Fig. 1 is a central vertical sectional view through a photo sensitivetube;

Fig. 2 is a similar view, partly broken away, taken on the line 2 ofFig. 1;

Fig. 3 is an enlarged section through the cathode, taken on the line 3-3of Fig. 2;

Fig. 4 is a diagrammatic fragmentary perspective view of another form ofwire cathode: and

cation having wires.

One form of photo cell construction comprises my new cathode l0associated with an anode ll mounted within a glass tube or transparentcasing I 2 of suitable shape. This light transparent tube may comprise abase ll of a soft lime glass or other suitable material which isintegral with the cylindrical tube walls. The vertical anode wire II, ora rectangular wire frame similarly used for the anode, may have asupporting wire portion I5 embedded in and projecting through the glassbase I4, and it is connected within the glass to a suitable wire lead l6which in turn connects with the metal prong I! carried bythe Bakelite orother insulating base ll! of the tube. Such tube and anode parts may beof suitable or standard constructions.

In the tube form shown, a shield IQ of a suitable opaque or lightfiltering medium is arranged in front of the cathode so as to shadow it.It extends half way, or otherwise as desired, up the length of thesensitized portion of the cathode ID. This shield, as described in mypatents, may be the cathode formed of separate an outside plate or tubespaced from the glass casing l2, or, as shown, a.coating on the glass 12which provides for shadowing a varying area of the cathode surface as alight ray moves angularly from the position a of zero illumination tothe position b of full illumination of the cathode, as indicated inFig. 1. The tubular shield [9 on the glass tube 12 may comprise agelatin film, such as the Wratten gelatin filters identi-' fiedcommercially by the numbers 64, and 6'1 which transmit light other thanred and infrared.

The transluscence of that filter to other light rays may be controlledby the addition of a 'dye or an opaque powder, such as dispersedgraphite.

Likewise, this shield may be made of other suit-- that will prevent thepassage of selected wave lengths of light.

My cathode, in the form shown in Figs. 1 and 2, comprises a rectangularmetal wire frame20. of suitable size having a downwardly project ingcentrally located metal support 2| formedpreferably of the same type ofwire. The support 2| 'in turn connects within the glass [4 with alead-in wire 22 which terminates in the metal Fig. 5 is a fragmentaryelevation of a modifl- I prong 23 carried by the base. The cup-shapedinsulating base 18 is suitably secured to the lower end of the glasstube 1:2 and. holds it properly in place, as will be understood. Thecentral hollow projection 25 of the glass tube I2 is for the purpose ofexhausting the air from the inside of the tube after which the hole inthe part 25 is sealed:

The primary feature of this invention comprises making the lightsensitive portion of the cathode of a coated wire 30 mounted on theframe 20, or other support, such a glass plate of rectangular or otherdesired shape. wire is connected to the terminal 23 through the Thismetal frame 20 or separately ii mounted on insulating material.

of parallel sections, either spaced or in Contact.

It is preferred that the wire be continuous and arranged in verticalloops. The wire elements may; however, be formed of separate wires soldered or otherwise secured to the metal frame:

and suitably connected to the outside termirials. Where one wire sectionis back of another,

it is preferred that the loops be vertical relative to the angulardirection of the light movement,

so that no material front portion of the wire. will cast a shadow on anyrear part of required.

use and thus affect the electron emission. As shown inFig} 2, thecathode frame may com= prise "a separate lower cross bar 3! and an uppercross bar 32. The sensitiged wire, or a plurality of separate sectionsof wire, is secured to these two bars 3| and 3 2, such as by wrapping asingle wire thereabout, as shown in Fig. 3, with the evenly spaced backwire elements showing be tvveen the evenly spaced front wire elements.The spacing between the adjacent w'ire elements is preferably aboutequal to the diameter or the wire, so that no material amount of light(in dicated by the arrows in Fig. 3) is' lost by pas-.

sage through the wire assembly; Various etipedients may be adopted forwrapping a wire continuously around the frame and insuring that.

the light exposed elements are parallel and vernear The wire flame 20with its cross bars a: and 3! may be made of a suitable metal, such as;nickel,- platinum, tungsten or 'a tungsten molyb,-,

denum alloy, or other strong and electrically conductive material whichwill properly support thc'cathode wires. These wire elements 30 arepreferably made of nickel coated with silyer, and they may be suitablysecured to the;

frame by a silver or other type of solder; or the frame may havelocating ribs or depressionsar=-;

ranged to position the wire 3;) and insure a dennitespaeing; A preferredlight sensitive medium is formed of silver coated with caesium metal ona base of oxidized caesium which is so treated as to provide asensitized surface in accorda'nce with standard practice, so as to form;

a silver-caesium oxide-caesium element;

One suitable form oi cathode may be made ot a nickel wireelectroplatedlightly with copper; and then with a coating of silver. The framed-0 mayalso be madeof this same materiai. The

it is to be sensitized after it is mounted in the a "silver solder.After the wire name inserted in the glass'tub'e with its lead wire'cOnwrres suitably'solderd to the frame, such as by.

nected thereto and in proper association with the anode, the gas in thetube made be exhausted by a vacuum system and other suitable proce dure.Thereafter, oxygen may be admitted to the tube in sufficient amount sothat when the cathode wire is electrically heated, the silver issuitably oxidized. Caesium is applied to this oxidized coating, such asby sdbliinihg the caesium metal derived from caesium trinitride on amagnesium ribbon or by decomposition of a pellet of caesium chromate andsilicon that has been suitably mounted in the tube for the purpose. Theapplication of heat decomposes the pellet and forms a vapor of caesiumwhich reacts with the silver oxidized coating to form caesium hiideinterspersed with the caesium metal when the wire is baked at about 200C. or higher. Ulti'mately the tube may be exhausted to provide thenecessary vacuum, or an inert gas, such as argon, may be added. Thisprocedure forms a cathode having a threshold at about 11 500 Angstromunits and a selective maximum at 6,070 A. If the oxidized coating andcaesium are applied to the wire before the latter is mounted on thesupporting frame, suitable precautions are taken to insure that thesensitivity of the coating remains or is restored. In that case; thesensitized wire is preferably wrapped about the frame 20 but-notsoldered to it. It will also be =ap= preciated that the other photo-sensitive materials, such as other alkali metals or alkaline earthmetals or combinations, such as antimonycaesium, may be appropriatelyprovided and treated. The various sensitive coatings may be formed bysuitable standard procedure.

If desired, as shown diagrammatically in Fig; 4, the cathode support maycomprise parallel rods 35 having one or more sensitized wires -38wrapped around it as a cylindrical helix or other shape, such as ahollow parallelopipedon or a flattened helix; and in such cases theanode may be arranged either within or outside of this catm ode shell.If the cathode is made like a woven screen of preferably fine mesh, thetwo of substantially parallel wire elements of the weave will lie incontact where they cross. However, for USES set Tfiith ii]. iii? filibf'patents, this Vfbical Wil arrangement Withoilt any cross WilES isdesirable, since the ever-13 m portions of crossed wires would throwsire-cows care the underlying sensitized portions of the cathode;

and the effect of ti'ise shadows Woifl'd vary with Changes in thdirective angularity of the light; Also, for many purposes the elementsof the loops or a single continuous wire wrapped around the frame 20need not be in exact parallelism but may be arranged siibstantiallyinparallelism as a normally wrapped helix, whether cylindrical or flat,with an equal progressive epaeing petwgeii the elements or loop portionsof the helix. The size i the cathode wire e emei 'ts and its sup=porting name may be varied widely, as win be understood. A wire of '15to 20 gauge will serve M the frame '29. am a wire or '20 to so gau canbe used for the silver caesium element. I have found that a cathode thatis suitable T61 the purposes of san parents may be non /g to or weldedby spot or resistance welding to a rectangular metal frame 4| andarranged in parallelism. These wires may be closely spaced, or they maybe in contact, so as to catch a desired proportion or substantially allof the light on the front exposed face portion of each wire element.Each of the wire elements is a single separate piece, and the elementsare not wrapped around the frame, as shown in Figs. 2 and 4. The anodeplate 42 is shown as a solid thin metal plate. It, however, may be madeas a' rectangular loop located outside of the path of the light raystowards the cathode wire elements. In such constructions where the anodeplate has a considerable area and the cell could not be used for highfrequency work due to its high capacitance, it is feasible to lower thatcapacity by the use of an opaque dielectric with its two vertical edgessupported by anode wires which replace the all metal sheet shown in theFig. 5 construction. Such an anode could be substituted for a centralflat metal plate used in the Fig. 4 construction within the loop of thecathode wire. It will be appreciated that many modifications may be madein this construction within the scope of this invention.

The advantages, utility and operation of such constructions will beapparent. A cylindrical wire will give a maximum surface effect, andthe.

cathode may be sensitized more uniformly for my purposes. If a spot onthe wire is not properly coated, this will be immaterial because of thesmall percentage of the ineffective area relative to the total cathodesurface. This cathode structure is such that the sum of the areas of thelight sensitive coatings of the wire elements which are exposedsimultaneously to the light constitute the effective activation area ofthe cathode, subject to the fact that the activation varies with thecosine of the angle of the impinging light. This activation issubstantially uniform and equal in equal unit areas of the light exposedcathode, without regard to the location of the area, so that theelectron emission varies according to the total area affected by thelight. Since the preferred arrangement of Fig. 2- has the front wireelements 30 spaced by about the thickness of a rear element andstaggered relative thereto, the assembly forms an apparently continuoussurface that catches most of the impinging light.

It should be understood that these special wire cathode constructionsneed not be used with the light shield l9 above described, and that thewire cathode is of general utility in many types of cell and for generalpurposes where the angle of the light direction is otherwise varied,

I claim:

1. A light sensitive photo cell comprising a light transparent casing, acathode having an extensive area of surface for light activation, anassociated anode and terminals leading from the cathode and anode, and ashield terminating in an edge which is mounted to shadow an area of thecathode that varies in accordance with the directive angularity of lightpassing said edge, said cathode comprising a metal supporting frame anda plurality of substantially parallel wire elements mounted on the framefor exposure to said light throughout the major extent thereof andcollectively connected to the cathode terminal, each wire element havinga light sensitive electron emissive coating, and the sum of the areas ofthe light sensitive coating exposed to directional light impingingthereon constituting the effective activation area of the cathode.

2. A photo cell according to claim 1 inwhich the cathode comprises ametal frame having a continuous coated wire wrapped therearound in loopswith the rear elements of the loop wire exposed to directional lightpassing between the front elements.

3. A photo cell according to claim 1 in which the cathode comprises ametal frame and a continuous light sensitive coated wire wrapped aroundthe frame and forming substantially parallel closely arranged front andrear elements, the spacing between adjacent front elements beingsubstantially the same as the width of a rear element and the front andrear elements being staggered so that the rear elements are exposed todirectional light passing between the front elements, whereby the frontand rear elements provide a light sensitive area substantially as largeas the area of the frame and form an apparently continuous surface forlight exposure.

4. A light sensitive photo cell comprising a light transparent casing, aframe mounted within the casing which has opposed supports bordering alight transmitting area, substantially parallel cathode elements of wiremounted on said opposed supports at closely spaced points and extendingacross said space, said wire having a light sensitive electron emissivecoating throughout its exposed length within the frame and an extensivecylindrical surface providing an effective activation area which is thesum of the coating areas exposed to directional light, the electronemission from the cathode elements being substantially uniform and equalin equal unit areas of the entire light activated area, an anodeassociated with the cathode in the casing, elec-, trical terminalsleading from the anode and the coated cathode wire elementsrespectively, and means which shadows a varying area of the sensitizedcathode wire elements in accordance with the directive angularity of thelight.

ROBERT J. HERBOLD.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,980,702 Salzberg Nov. 13, 1934 2,013,095 Friend Sept. 3,1935 2,056,967 Hopkins Oct. 13, 1936 2,185,395 Bamford Jan. 2, 19402,236,012 Shockley Mar. 25, 1941 2,307,697 McIlvaine Jan. 5, 19432,489,222 Herbold Nov. 22, 1949

